Method of forming GE photodetectors

ABSTRACT

A photodetector device includes a plurality of Ge epilayers that are grown on a substrate and annealed in a defined temperature range. The Ge epilayers form a tensile strained Ge layer that allows the photodetector device to operate efficiently in the C-band and L-band.

PRIORITY INFORMATION

This application claims priority from provisional application Ser. No.60/389,819 filed Jun. 19, 2002, which is incorporated herein byreference.

This application is a continuation of U.S. patent application Ser. No.10/307,779 filed on Dec. 2, 2002 now U.S. Pat. No. 6,812,495.

BACKGROUND OF THE INVENTION

The invention relates to the field of photodetectors, and in particularto monolithically integrated Ge photodetectors on Si.

Photodetectors are fundamental devices that convert optical signals intoelectric signals. Fiber optical communication employs 1300 and 1550 nmwavelengths because of low attenuation coefficients of silica fibers. Erdoped fiber amplifiers emphasize the importance of 1550 nm because ofthe direct amplification of optical signals without converting toelectric signals. The amplification range between 1530-1560 nm isreferred to as C-band, and the recently extended amplification rangebetween 1570-1608 nm is referred to as L-band. The photodetectors for1550 nm detection have so far been InGaAs photodetectors, since InGaAsis a direct semiconductor whose bandgap is 0.75 eV (corresponding to1653 nm). Thus, InGaAs photodetectors can convert any optical signal inthe C- and L-bands to electric signals. These optical fibercommunication components are well developed.

Recently, optical technology has expanded its territory from fibercommunication to photonic integrated circuits on a chip. This allows forhigh speed and broad band communication. The impact is even larger ifoptics is merged into Si LSIs, e.g., 10 GHz clock processors, etc.InGaAs photodetectors are not easy to implement on a silicon chip, sinceInGaAs is a III-V compound semiconductor. In general, the elements In,Ga, and As are all dopants in silicon to show donor or acceptorcharacteristics and could thus alter the circuit performance ifdiffused. Ge can be a candidate for on-chip photodetectors, since Ge isin the group IV element and produces no harmful effects if diffused.Thus, Ge provides a perfect opportunity to form highly efficientphotodetectors.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided aphotodetector device. The photodetector device includes a plurality ofGe epilayers that are grown on a silicon substrate and annealed in adefined temperature range. The Ge epilayers form a tensile strained Gelayer that allows the photodetector device to operate efficiently in theC-band and L-band.

According to another aspect of the invention, there is provided a methodof forming a photodetector device. The method includes forming aplurality of Ge epilayers that are grown on a substrate. Moreover, themethod includes annealing the Ge epilayers in a defined temperaturerange. Furthermore, the method includes developing a tensile strained Gelayer using the annealed Ge epilayers, the tensile strained Ge layerallowing the photodetector device to operate efficiently in the C-bandand L-band.

According to another aspect of the invention, there is provided aphotodetector device. The photodetector device includes a plurality ofSiGe epilayers that are grown on a substrate at a high temperature so asto form a SiGe structure. The SiGe layer forms a tensile strained SiGelayer by cooling to room temperature the SiGe structure using thebi-metal effect. The tensile strained SiGe layer allows thephotodetector device to operate efficiently in the C-band and L-band.

According to another aspect of the invention, there is provided a methodof forming a photodetector device. The method includes growing aplurality of SiGe epilayers on a silicon substrate at a high temperatureso as to form a SiGe structure. Furthermore, the method includes forminga tensile strained SiGe layer by cooling to room temperature the SiGestructure using the bi-metal effect. The tensile strained SiGe layerallows the photodetector device to operate efficiently in the C-band andL-band.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic block diagram illustrating the techniques used tocreate a Ge photodetector using a tensile strained Ge layer;

FIG. 2 is an energy band diagram of Ge under stress;

FIG. 3 is a graph of the absorption of a Ge epilayer grown using theinvention;

FIG. 4 is a graph of an optical bandgap of a Ge epilayer and a bulk Geanalyzed using photoreflectance spectroscopy;

FIG. 5 is a graph demonstrating the tensile strain in Ge shrinking itsdirect bandgap (Eg);

FIG. 6 is a X-ray locking curve measurement of the annealed tensile Geepilayer.

DETAILED DESCRIPTION OF THE INVENTION

Ge is an indirect semiconductor whose bandgap is 0.667 eV correspondingto 1850 nm. Because of indirect bandgap characteristics, the absorptioncoefficient is small, approximately 100 cm⁻¹ depending on thewavelength. The direct bandgap of Ge is 0.8 eV corresponding to 1550 nm.Thus, the photodetector performance of Ge is marginally good in theC-band but, is poor in the L-band. To overcome the drawback of Ge,various attempts have been used, such as GeSn alloy, to decrease thebandgap.

One goal in Ge photodetectors is to shrink the bandgap to cover the C-and L-bands with high efficiency. The invention uses tensile strain toaccomplish this task.

FIG. 1 is schematic block diagram illustrating the techniques used tocreate a Ge photodetector 2 using a tensile strained Ge layer 8. The Gephotodetector is formed on a Si wafer. This is ideal for realizingSi-based optoelectronic circuits as well as optical interconnection inSi-LSIs. In spite of a large lattice mismatch (4%) between Ge and Si, alayer 8 of Ge is grown directly on the Si wafer 4 with uniform thicknessand with low density of threading dislocations, using anultrahigh-vacuum chemical vapor deposition (UHV-CVD) technique. Otherdepositing techniques can be used in accordance with this invention.

By growing the Ge layer 8 directly on the Si, the bandgap shrinkage isshown to be induced by tensile strain. The Ge layer 8 includes a definednumber of Ge epilayers 6, grown at 700° C. on the heavily-doped p-Si(100) wafer using the UHV-CVD technique, which also uses GeH₄ (15% inAr) as the deposition gas. After the growth, cyclic thermal annealing(10 cycles between 900° C. and 700° C.) is performed to reduce thedislocation density. Absorption coefficients derived from responsibilityspectra indicate substantial red shift compared with bulk Ge. It isshown from PR spectra that the red shift is induced by bandgapshrinkage. The Ge epilayers show approximately 0.77 eV, which is smallerby approximately 0.03 eV than direct bandgap for bulk Ge. This propertyis beneficial for Ge photodetectors that can operate in the C-band aswell as the L-band. The origin of the shrinkage is found to be not byelectric-field-enhanced tunneling such as Franz-Keldysh effect. Thepseudo potential calculation suggests that the bandgap shrinkage wouldoccur under tensile strain of approximately 0.2%. It is discussed thatgeneration of such tensile strain is ascribed to the difference ofthermal expansion between Ge and Si.

Furthermore, this method is applicable for not only Ge epilayers 6, butalso SiGe where Ge concentration is more than 50% to introduce thermalmismatch. In the case of SiGe, the equilibrate temperature should bedesigned higher than 900° C. and preferably close to the meltingtemperature of the alloy. In this embodiment, Si is used as a substrate,however, other types of substrates can be used whose lattice expansioncoefficient is smaller that Ge.

Another technique in creating tensile stress in Ge is to use the thermalmismatch between Ge and Si. The linear expansion coefficients of Ge andSi are 5.8×10⁻⁶ K⁻¹ and 2.6×10⁻⁶ K⁻¹, respectively. When the Geepilayers 6 are grown on Si and then heat up to a high temperature aftergrowth to equilibrate these lattices so as to have its own latticeconstants, the tensile strain should be introduced in the Ge epilayers 6by cooling to room temperature the SiGe structure in terms of the socalled bi-metal effect.

According to this technique, isothermal annealing at 900° C. for 10 minor longer for Ge epilayers grown at 700 C is performed to fully relaxthe Ge lattice constant; Ge has its own lattice constant. Alsoisothermal annealing at 900° C. for 30 min or longer for Ge epilayersgrown at 600° C. is needed to fully relax the Ge lattice constant. FIG.5 shows the theoretical limit of the bandgap shrinkage using thebi-metal effect. It is assumed that rapid thermal annealing is used tocool the Ge epilayers instantaneously to room temperature. Thistechnique of rapid cooling of the Ge epilayer on Si should be moreeffective than slow cooling, since lattice relaxation could happen inslower cooling. It shows the maximum strain 0.32% and the minimumbandgap 0.757 eV as shown in FIG. 2. It is marked that the bandgapcorresponds to 1638 nm is much longer than the long wavelength edge ofthe L-band.

SiGe epilayers can also be grown using this technique. The isothermalannealing temperature can be approximately 1330° C. for Si_(0.5)Ge_(0.5)epilayers.

FIG. 2 is an energy band diagram of Ge under stress. Pseudo potentialcalculations have been used. The band diagram shows that the Ge bandgapshrinks when tensile stress is induced. The Ge direct bandgap consistsof G valley and degenerated hole bands. When the tensile stress isintroduced, the energy position of the G valley is lowered and that ofthe light hole band is higher, eventually the direct bandgap shrinks.FIG. 2 also shows that the tensile stress of 0.2% is needed to shift theband edge up to 1600 nm to cover the L-band. As noted above our maximumstrain is 0.32% and the band gap is 1638 nm which should cover the Lband.

FIG. 3 is a graph of the absorption of a Ge epilayer grown using theinvention. In particular, the graph shows the absorption coefficients ofthe Ge epilayer grown using the invention and bulk Ge. Furthermore, thegraph shows increase sensitivity up to 1600 nm between the C-band andL-band for the Ge epilayer. The Ge bulk does not demonstrate increasesensitivity in the C-band and practically no sensitivity in the L-band.Thus, using Ge epilayers as a tensile Ge layer can be beneficial foroperations in the C-band and L-band with sufficient sensitivity. Usingrapid thermal annealing the Ge epilayers can be cooled rapidly, leadingfurther shrinkage of the bandgap.

FIG. 4 is a graph of an optical bandgap of a Ge epilayer and a bulk Geanalyzed using photoreflectance spectroscopy. The vertical axis isassociated with bandgap energy and the horizontal axis is associatedwith the a constant value C_(j)=[3π(j−½)/2]^(2/3), where j is an integervalue that denotes the number of peaks and valleys in the spectrumcounted from the longer wavelength. Overall, the graph demonstrates thata Ge epilayer grown at 700° C. and then heated up to 900° C., using theannealed steps described previously, and cooled rapidly shows a narrowerbandgap than bulk Ge. Furthermore, the graph illustrates throughout alarge span of j values (1-5), the bandgap is consistent with a structurein the C-band and L-band and rarely fluctuates over the 0.75 eV bandgap.The bulk Ge structure shows a structure that fluctuates across a smallrange of j values (1-3), thus not making this structure useful in theC-band and L-band.

FIG. 5 is a graph demonstrating the tensile strain in Ge shrinking itsdirect bandgap (Eg). The theoretical limit of Eg shrinkage is 0.757 eVcorresponding to 1638 nm, which is beyond the L-band edge (1608 nm) ofEr amplifiers. The maximum strained accumulated during the coolingprocess previously discussed is 0.0032 based on the reported expansioncoefficients of Ge and Si. The tensile strain of Ge can function as auniversal photodetector for the S+C+L bands of Er doped amplifiers. Thesame properties can also be seen using a tensile strain SiGe layer.

FIG. 6 is a X-ray locking curve measurement of the annealed tensile Geepilayer. The X-ray locking measurement includes both a tensile Geepilayer and an unannealed Ge layer, and measures the wavelength of anX-ray arriving at the sampled at a specified angle. This establishes thelattice constant of the samples. FIG. 5 shows that the lattice constantof the Ge epilayer is indeed larger than bulk Ge by 0.2%.

Another technique to more effectively utilize the thermal mismatch inthe Ge epilayers is to grow Ge on a substrate called “Si on Quartz”.Since the lattice constant expansion coefficient of quartz is one orderof magnitude smaller than Si, larger strain should be accumulated in theGe epilayers. The invention is also applicable for SiGeC as well.

Furthermore, another advantage of this tensile strained Ge is its lighthole band. The valence band now consists of light hole bands whosedensity of state is much smaller than the heavy hole bands. This resultsin lower saturation current in diode reverse characteristics, which areuseful in photodetectors devices, and a faster drift velocity of holes.This is beneficial in the Ge photodetectors operating in the driftlimited regime, not in RC limited regime. In the drift-limited regime,the device response time is limited by slow holes. However, light holeis as fast as electron in the F point, so the response time becomes theshortest.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1. A method of forming a photonic device, the method comprising: growinga plurality of Ge epilayers at a high growth temperature on a substratehaving a lattice expansion coefficient smaller than a lattice expansioncoefficient of Ge; annealing the Ge epilayers at an annealingtemperature; and cooling the Ge epilayers to room temperature so as toform a tensilely strained Ge layer.
 2. The method of claim 1, whereinthe growth temperature is greater than 600° C.
 3. The method of claim 2,wherein the growth temperature is greater than 700° C.
 4. The method ofclaim 1, wherein growing the Ge epilayers comprises UHV-CVD.
 5. Themethod of claim 4, wherein the UHV-CVD growth comprises use of GeH₄ as adeposition gas.
 6. The method of claim 1, wherein annealing comprisescyclic thermal annealing.
 7. The method of claim 6, wherein the cyclicthermal annealing is performed between annealing temperatures 700° C.and 900° C.
 8. The method of claim 1, wherein the annealing temperatureis higher than the growth temperature.
 9. The method of claim 8, whereinannealing the epilayers comprises isothermal annealing.
 10. The methodof claim 9, wherein the annealing temperature during the isothermalannealing is about 900° C.
 11. The method of claim 1, wherein coolingthe epilayers comprises rapid cooling.
 12. The method of claim 1,wherein the substrate comprises silicon.
 13. The method of claim 12,wherein the substrate comprises heavily-doped p-type silicon.
 14. Themethod of claim 1, wherein the substrate consists essentially ofsilicon.
 15. The method of claim 1, wherein the photonic devicecomprises a photodetector device.
 16. The method of claim 1, wherein thetensile strain of the Ge layer induces bandgap shrinkage of the Gelayer.
 17. A method of forming a photonic device, the method comprising:growing a plurality of Ge epilayers at a high growth temperature on asubstrate having a lattice expansion coefficient smaller than a latticeexpansion coefficient of Ge; annealing the Ge epilayers at an annealingtemperature; and cooling the Ge epilayers to room temperature so as toform a tensilely strained Ge layer, wherein the tensilely strained Gelayer shows bandgap shrinkage under tensile strain between about 0.2%and 0.32%.
 18. A method of forming a photonic device, said methodcomprising: growing a plurality of epilayers comprising SiGe at a highgrowth temperature on a substrate having a lattice expansion coefficientsmaller than a lattice expansion coefficient of Ge; annealing theepilayers at an annealing temperature; and cooling the epilayers to roomtemperature so as to form a tensilely strained layer comprising SiGe.19. The method of claim 18, wherein the epilayers consist essentially ofSiGe.
 20. The method of claim 19, wherein the SiGe epilayers comprise aGe concentration of at least 50%.
 21. The method of claim 18, whereinthe epilayers comprise SiGeC epilayers.
 22. The method of claim 18,wherein the substrate comprises silicon.
 23. The method of claim 18,wherein the substrate consists essentially of silicon.
 24. The method ofclaim 18, wherein the growth temperature is greater than about 600° C.25. The method of claim 24, wherein the growth temperature is greaterthan about 700° C.
 26. The method of claim 18, wherein growing theepilayers comprises UHV-CVD.
 27. The method of claim 18, whereinannealing the epilayers comprises isothermal annealing.
 28. The methodof claim 27, wherein the annealing temperature during the isothermalannealing is about 1330° C.
 29. The method of claim 18, wherein coolingthe epilayers comprises rapid cooling.
 30. The method of claim 18,wherein the photonic device comprises a photodetector device.
 31. Themethod of claim 18, wherein the tensile strain of the SiGe layer inducesbandgap shrinkage of the SiGe layer.
 32. A method of forming a photonicdevice, said method comprising: growing a plurality of SiGe epilayers ata high growth temperature on a substrate having a lattice expansioncoefficient smaller than a lattice expansion coefficient of Ge;annealing the SiGe epilayers at an annealing temperature; and coolingthe SiGe epilayers to room temperature so as to form a tensilelystrained SiGe layer, wherein the tensilely strained SiGe layer showsbandgap shrinkage under tensile strain between about 0.2% and 0.32%.